As shown in Scheme 1, muconic acid (“MA”) is an unsaturated dicarboxylic acid, hexe-2,4-dienedoic acid, which can exist in three isomeric forms.

Muconic acid has garnered significant interest due to its potential use as a platform chemical for the production of several valuable consumer bio-plastics including nylon 6,6, polyurethane (via an adipic acid intermediate), and polyethylene terephthalate (PET) (via a terephthalic acid intermediate). Adipic acid is produced mainly from petrochemicals like benzene. Because of the strong environmental impact of the production processes and the dependence on fossil resources, biotechnological production processes have been extensively explored.
For example, C. Weber et al., Appl. Env. Microbiol., 71, 8421 (2012) disclose the use of engineered Saccharomyces cerevisiae strain expressing a heterologous biosynthetic pathway converting glucose into cis,cis-muconic acid via the intermediate 3-dehydroshikimate of the aromatic amino acid biosynthesis pathway via protocatechuic acid and catechol, which can potentially be chemically hydrogenated to adipic acid. K. A. Curran et al., Metab. Eng., 15, 55 (2013) have reported a three-step synthetic, composite pathway including importing the enzymes dehydroshikimate dehydratase from Podospora anserina, protocatechuic acid decarboxylase from Enterobacter cloacae, and catechol 1,2-dioxygenase from Candida albicans into yeast. Further genetic modifications guided by metabolic modeling and feedback inhibition mitigation were introduced to increase precursor availability that led to a final titer of nearly 141 mg/L muconic acid in a shake-flask culture, a value nearly 24-fold higher than obtained from the initial strain.
Furthermore, the hydrogenation of muconic acid to adipic acid presents many of the difficulties encountered in the hydrogenation of edible oils and fats. The majority of commercially hydrogenated oils and fats are processed with batch reactor equipment using high temperatures, chemical catalysts, and hydrogen gas supplied to the reactor at elevated pressures. The hydrogenation catalysts used include Raney and supported nickel catalysts, promoted nickel catalysts containing palladium, copper, or zirconium, and copper chromite catalysts. The rate of hydrogenation is dependent on the reaction temperature, the nature of the oil or fat, the activity and concentration of the catalyst, and the rate at which hydrogen gas and unsaturated oil or fat are supplied to the hydrogenation reactor. Typical reaction pressures and temperatures are in the range of 10-60 bar and 150°-225° C., respectively. These elevated temperatures and pressures are required to solubilize sufficiently high concentrations of hydrogen gas in the oil/catalyst or fat/catalyst reaction medium so that the hydrogenation reaction proceeds at acceptably high rates.
Electrocatalytic hydrogenations of unsaturated organic compounds using Raney nickel or similar low hydrogen overvoltage catalysts as cathode materials can employ less rigorous conditions, and have been reported by a number of investigators (e.g., T. Chiba et al., Bull. Chem. Soc. Jpn., 56, 719 (1983); L. L. Miller et al., J. Org. Chem., 43, 2059 (1978); L. V. Kirilyus et al., Sov. Electrochem., 15, 1330 (1979); K. Park et al., J. Electrochem. Soc., 132, 1850 (1985)). These studies have dealt with the electrochemical hydrogenation of unsaturated hydrocarbons, phenols, ketones, nitro-compounds, and sugars rather than unsaturated fatty acids. Pintauro (U.S. Pat. No. 5,225,581) discloses a two-phase electrocatalytic process for hydrogenating unsaturated fatty acids or triglycerides using hydrogen generated electrolytically on a high surface area, low hydrogen overvoltage catalytic material used as the cathode. However, this process was not disclosed to be useful to hydrogenate unsaturated alkene dioic acids, either in purified form, or in situ as formed in the fermentation broth used in their biosynthesis from organic precursors.